A variety of experimental methods have been developed to measure the shear rheology of surfaces and interfaces (Murray and Dickinson 1996). One of the most commonly used methods is analogous to the concentric cylinder technique used to measure the shear properties of bulk materials (Chapter 8). The sample to be analyzed is placed in a thermostated vessel, and a thin disk is placed in the plane of the interface that separates the two phases (e.g., water-and-air or oil-and-water) (Figure 5.20). The vessel is then rotated and the torque on the disk is measured. The sample can be analyzed in a number of different ways depending on whether it is solid-like, liquid-like, or viscoelastic. For liquid-like interfaces, the torque on the
disk is measured as the vessel is rotated continuously. For solid-like interfaces, the torque is measured after the vessel has been moved to a fixed angle. For viscoelastic interfaces, the torque is measured continuously as the vessel is made to oscillate backward and forward at a certain frequency and angle.
The interfacial shear viscosity or elasticity of surfactant membranes is usually several orders of magnitude less than that of biopolymer membranes because biopolymer molecules often become entangled or interact with each other through various covalent or physical forces. The rheology of bulk emulsions depends on the concentration, size, and interactions of the droplets. By analogy, the rheology of interfaces depends on the concentration, size, and interactions of the adsorbed emulsifier molecules. Interfacial shear rheology measurements are particularly useful for providing information about adsorption kinetics, competitive adsorption, and the structure and interactions of molecules at an interface, especially when they are used in conjunction with experimental techniques that provide information about the concentration of the emulsifier molecules at the interface (e.g., interfacial tension or radioactive labeling techniques). The concentration of emulsifier molecules at an interface often reaches a constant value after a particular time, while the shear modulus or viscosity continues to increase because of interactions between the adsorbed molecules (Dickinson 1992).
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